It is well-known that one can realize an electric micromotor with a rotor comprising a magnetic circuit constituted by one or a plurality of permanent magnets which, when the rotor turns, creates an alternating magnetic field and induces a voltage of the terminals of one or a plurality of fixed coils situated in the air gaps of said alternating magnetic circuit. The speed of rotation is maintained by the electrical energy furnished by an electronic control circuit which delivers current impulses in phase with the induced voltage.
This type of motor can be synchronized in such manner as to rotate in synchronism with a reference frequency, delivered, for example, by a quartz oscillator and a frequency divider while proportioning this energy by means of a regulating circuit.
If it is desired to obtain a high output, it is necessary to create an alternating field of the highest possible frequency to have an elevated value of (d.phi. /dt), and therefore, a substantial induced voltage. This is indispensable especially in applications for watches where the volume and the output play a very great role.
In the rotors of known construction, magnets of opposite polarity are placed very close to one another and must be magnetized in place, as it is practically impossible to mount premagnetized magnets in mass production. This operation is long, therefore costly, and generally only one piece can be magnetized at a time.